# ARC OF VISIBILITY

The portion of the horizon over which a lighted aid to navigation is visible from seaward.

## Related Terms

### HORIZON SYSTEM OF COORDINATES

A set of celestial coordinates based on the celestial horizon as the primary great circle; usually altitude and azimuth or azimuth angle.

Navigation by the use of grid directions

### HORIZON

The great circle of the celestial sphere midway between the zenith and nadir, or a line resembling or approximating such a circle. The line where earth and sky appear to meet, and the projection of this line upon the celestial sphere, is called the visible or apparent horizon. A line resembling the visible horizon but above or below it is called a false horizon. The circle of the celestial sphere-formed by the intersection of the celestial sphere and a plane perpendicular to the zenith-nadir line is called sensible horizon if the plane is through any point, such as the eye of an observer; geoidal horizon if through any sea-level point; and celestial or rational horizon if through the center of the earth. The geometrical horizon was originally considered identi- cal with the celestial horizon, but the expression is now more commonly used to refer to the intersection of the celestial

### HOMING

Navigation toward a point by following a signal from that point. Radiobeacons are commonly used for homing.

### HORIZON MIRROR

The mirror part of the horizon glass. The expression is sometimes used somewhat loosely to refer to the horizon glass.

### RUST PREVENTION TEST

A test for determining the ability of an oil to aid in preventing the rusting of ferrous parts in the presence of water.

### HORIZON GLASS

The glass of a marine sextant, attached to the frame, through which the horizon is observed. The half of this glass nearer the frame is silvered to form the HORIZON MIRROR for reflecting the image of a celestial body; the other half is clear.

### INERTIAL ALIGNMENT

The process of orienting the measuring axes of the inertial components of inertial navigation equipment with respect to the coordinate system in which the equipment is to be used. inertia

### INLAND RULES OF THE ROAD

Officially the Inland Navigation Rules; Rules to be followed by all vessels while navigating upon certain defined inland waters of the United States.

### INDEX ERROR

The error in the reading of an instrument equal to the difference between the zero of the scale and the zero of the index. In a marine sextant it is due primarily to lack of parallelism of the index mirror and the horizon glass at zero reading.

### Related questions

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